adaptive control

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ADAPTIVE CONTROL SYSTEM IN MACHINING Presented by SAMPATH KUMAR R S (MDM13B026) SNEHIL WANKHEDE (MDM13B027) SOWMYA ROKKAM (MDM13B028) SANDEEP SUDDARSI (MDM13B029)

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this ppt is about adaptive control machining

Transcript of adaptive control

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ADAPTIVE CONTROL SYSTEM IN MACHINING

Presented bySAMPATH KUMAR R S (MDM13B026)SNEHIL WANKHEDE (MDM13B027)SOWMYA ROKKAM (MDM13B028)SANDEEP SUDDARSI (MDM13B029)

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INTRODUCTIONAdaptive control system is a logical extension of the

CNC-mechanism. In CNC mechanism the cutting speed and feed rates are prescribed by the part programmer. The determination of these operating parameters depends on the Knowledge and experience regarding the work piece, tool materials, coolant conditions and other factors.

By contrast in adaptive control machining, there is improvement in the production rate and reduction in the machining cost as a result of calculating and setting of optimal parameters during machining.

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Adaptive control possesses attributes of both feedback control and optimal control. Like a feedback system measurements are taken on certain process variables. Like an optimal system, an overall measure of performance is used. In adaptive control, this measure is called the index of performance (IP).

An adaptive control system is designed to operate for the changing environment by monitoring its performance and altering accordingly, some aspects of its control mechanism to achieve optimal or near optimal routine.

INTRODUCTION

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FUNCTIONS OF ADAPTIVE CONTROL The three functions of adaptive control are:

1) Identification function

2) Decision function

3) Modification function.

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IDENTIFICATION FUNCTIONS This involves determining the current performance of the

process or system.

Normally, the performance quality of the system is defined by some relevant index of performance. The identification function is concerned with determining the current value of this performance measure by making use of the feedback data from the process.

Since the environment will change overtime, the performance of the system will also change. Accordingly the identification is one that must proceed over time or less continuously.

Identification of the system may involve a number of possible measurements activities.

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DECISION FUNCTIONOnce the system performance is determined,

the next function is to decide how the control mechanism should be adjusted to improve process performance. The decision procedure is carried out by means of a pre-programmed logic provided by the designer.

Depending upon the logic the decision may be to change one or more of the controllable process.

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MODIFICATION FUNCTION The third AC function is to implement the decision. While the

decision function is a logic function, modification is concerned with a physical or mechanical change in the system. It is a hardware function rather than a software function.

The modification involves changing the system parameters or

variables so as to drive the process towards a more optimal state. The process is assumed to be influenced by some time varying environment.

The adaptive system first identifies the current performance by taking measurements of inputs and outputs.

Depending on current performance, a decision procedure is carried out to determine what changes are needed to improve system performance. Actual changes to the system are made in the modification function.

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Figure below illustrates the sequence of the three functions in an adaptive controller applied to a hypothetical process

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ADAPTIVE CONTROL SYSTEMS The adaptive control of metal working process is a

logic extension of numerical control and computer control.

The main idea of AC is the improvement of the cutting process by automatic on line determination of speed and/or cutting.

The AC is basically a feedback system in which cutting speed and feed automatically adapt themselves to the actual condition of the process and are varied accordingly to the changes in the work conditions as work progresses.

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TYPES OF ADAPTIVE CONTROL

There are basically two groups of adaptive control:

Geometrical adaptive control.

Technological adaptive control.

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GEOMETRICAL ADAPTIVE CONTROLThis type of adaptive control is concerned with

monitoring the shape and dimension of a machined component

The index of performance of such a system is the final dimension and shape of the component.

It may be said that the desired final goal of the automated machining process is that the drawing of the work piece to be machined component is the output.

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TECHNOLOGICAL ADAPTIVE CONTROL

Under the umbrella of technological AC two distinct systems are grouped:

Adaptive control constraint(ACC)

Adaptive control optimization(ACO)

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AC with constrains (ACC)

ACC are systems in which machining conditions such as spindle speed or feed rate are maximized with in the prescribed limits of machines and tool constrains.

The most useful constraints in practical ACC systems are the cutting force F, the cutting power P, and the torque T.

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When the cutting force and torque are too high, the cutter may break in bending or in twist. On the other hand, for maximum productivity, the maximum allowable feeds and cutting speed must be used.

The principle of ACC system is to sense the constraint variable and to adjust the cutting speed and feed according to this measurement and a programmed strategy.

AC with constrains (ACC)

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In a case where two constraints are used simultaneously, the system must consider the variable which to its maximum permissible value.

The advantage of this ACC system is that the cutting tool is protected against catastrophic failure simultaneously with keeping highest possible feed rate.

AC with constrains (ACC)

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The position and speed commands are fed by the NC system to drive the machine. The resulting motion is sensed by feedback devices and sent back to the NC system to complete a servo loop.

The adaptive controller operates as a second feedback loop outside the servo loop. The signals from sensors mounted on the machine are fed into the adaptive control.

The latter determines, according to programmed constraints and its strategy, the appropriate corrections in the feedrate and spindle speed and sends the results to the AC system.

AC with constrains (ACC)

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The adaptive controller is fed by signals of two sensors:

1) Spindle torque sensor – The spindle torque is measured by strain-gauges mounted on the machine spindle.

2) Tool vibration sensor – The tool vibration is measured by two accelerometers mounted on the machining spindle housing.

AC with constrains (ACC)

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The sensor data are fed to the adaptive control unit which calculates new feed and speed to optimize metal removal rate within the limits of a set of constraints. These constraints are as follows:

Maximum permissible spindle speed in rpmMinimum spindle speedMaximum allowed torqueMaximum allowed chip load in /revMinimum chip loadMaximum permitted feedrate in rpmMaximum allowed vibrationImpact chip load, in in/rev

AC with constrains (ACC)

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AC with optimization (ACO)ACO refers to systems in which the

performance is optimized according to a prescribed IP(Index of Performance), sometimes called the figure of merit.

The ACO Systems for N/C machine tools is a control system that optimizes performance index subjects to various constraints. It is basically a sophisticated closed loop control system, which automatically works in optimum conditions

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The input of the adaptive controller is the data reduction Subsystem (DRS). This unit is fed by the online measurement as well as by the actual feed and speed and a set of constraints. The DRS produces two signals:

Tool wear Rate (TWR)

Metal Removal Rate (MRR).

AC with optimization (ACO)

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The TMR and MRR signals are fed in to a performances computer unit which calculates the performance index based on various factors such as cost of the tool, cost of the machine, etc.

The calculated IP is fed into an optimization computer unit (OCU), which contains the strategy according to which the optimization is made.

AC with optimization (ACO)

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AC with optimization (ACO)Thus the objective of this unit is to

continuously maintain the value of IP at the highest possible values without causing any constraint violation.

The constraints may be on our case: maximum spindle speed, minimum spindle speed, maximum torque, maximum feed, minimum temperature, maximum vibration amplitude, etc.

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WHERE TO USE ADAPTIVE CONTROLThe following are the typical sources of variability in

machining where AC can be most advantageously applied:

1) Variable geometry of cut in the form of changing depth or width of cut: In these cases, feed rate is usually adjusted to compensate for the variability.

2) Variable work piece hardness and Variable machinability: When hard spots or other areas of difficulty are encountered in a work piece, either speed or feed is reduced to avoid premature failure of tool.

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WHERE TO USE ADAPTIVE CONTROL

3) Variable work piece rigidity: if the work piece deflect as a result of insufficient rigidity in set up, the feed rate must be reduced to maintain accuracy in the process.

4) Tool wear: It has been observed in research that as the tool begins to dull, the cutting force increases. The adaptive controller will typically respond to tool dulling by reducing feed rate.

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ADVANTAGES OF AC SYSTEMSIncreased production rates.

Increased productivity. Increased tool life. Increased accuracy by making tolerance as a constraint.

Maximum component safety. Minimum operator intervention. Easy part programming.

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LIMITATIONS OF AC SYSTEMS

The lack of a tool wear sensor is the major obstacle to the employment of ACO systems.

Unavailability of standardized interfaces of an AC system with CNC units.

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CONCLUSIONSFor a machining operation adaptive control is the

best method for controlling the process. AC systems are best suited for applications like work pieces with variable geometry of cut in the form of changing depth and width of cut, air gaps etc.

But the progress in areas such as development of reliable sensors, machine tool design based on requirements of AC systems, and development of stable adaptive control strategies are required for efficient use in all the industries.